One of the requirements for long term evolution advanced (LTE-A) is to increase spectral efficiency over existing long term evolution (LTE) wireless systems. In order to fulfill the requirement, there are several candidate techniques considered for LTE-A, including single-user/multi-user (SU/MU) multiple-input multiple-output (MIMO), multi-site MIMO, relaying, and the like. Such techniques typically require control signaling associated with the respective technique in uplink (UL), downlink (DL), or both. For example, in a closed loop MIMO system, (e.g., a precoding MIMO system), a receiver sends channel state information (CSI), (i.e., a precoding matrix indicator (PMI)), in a control channel to a transmitter. Then, the transmitter signals to the receiver the precoding vector or matrix information which it uses for transmission precoding. Since this overhead signaling causes spectral efficiency to be reduced, it is desirable to enhance the control signaling technique.
In LTE-A, there may be some cases, techniques, or applications where there is information exchanged between wireless transmit receive units (WTRUs), and evolved Node-Bs (eNodeBs), or relays. For example, in multi-cell cooperative MIMO or MU MIMO in DL, it would be beneficial in the receiver processing if WTRUs exchange their CSI with each other, or if the eNodeB transmits to each of the WTRUs the precoding vectors or matrices applied not only for the respective WTRU but also for other WTRUs, so that each of the WTRUs can use all of the precoding vectors or matrices. However, in this case, the signaling overhead with the legacy LTE signaling scheme may be too excessive to implement.
Thus, it is desired to apply the network coding model in few technology components considered for LTE-A. In order to make the signaling more efficient, the network coding is applied to MIMO related control signaling.
Network coding has been considered as a particular in-network data processing technique that exploits the characteristics of the broadcast communication channel, in order to increase the capacity of the throughput of the network.
FIG. 1 depicts a canonical example of network coding. A source, S, sends data, b1 and b2, to nodes, T and U, respectively, each of which multicasts the received data to other nodes, W, Y and Z. At node W, the received data, b1 and b2, are encoded using network coding and forwarded to node X after applying an XOR operation. Node X then receives the network coded data, b1⊕b2, and multicasts a copy of the network coded data to nodes Y and Z.
Through this network information flow, the nodes Y and Z may receive both b1 and b2 without additional channel resources.
Network coding may be applied in MU MIMO control signaling. FIG. 2 shows a conventional wireless communication system 200 including an eNodeB 205 and a plurality of WTRUs 2101 and 2102.
In MU MIMO, multiple WTRUs 210 are paired (or scheduled), and they are served simultaneously by the eNodeB 205 via MU spatial multiplexing MIMO. That is, the eNodeB 205 simultaneously transmits to the WTRUs 210 through a MU MIMO technique, such as zero-forcing (ZF) MU MIMO. In this case, a signal intended to one of the (paired) WTRUs 210 would be interference to the receiver of another WTRU 210, unless orthogonality of the signals is not maintained at the receiver. To eliminate such interference at a paired WTRU receiver, it is desirable for the eNodeB 205 to signal the beamforming vectors used in MU MIMO to the respective paired WTRUs 210. Regarding the selection of paired WTRUs 210, the eNodeB 205 typically selects WTRUs 210 experiencing similar channel quality, but having CSI that is uncorrelated to each other.
In DL MU MIMO, (e.g., ZF MU MIMO or unitary based MU MIMO), the individual paired WTRUs 2101 and 2102 typically feed back their own CSI, in a form of codebook, PMI, or quantized CSI, to the eNodeB 205. Upon reception of the CSI, the eNodeB 205 determines the beamforming (BF) vectors or precoding matrices used for MU MIMO transmission, and then the eNodeB 205 uses two separate physical downlink control channels (PDCCHs) 2151 and 2152 to separately signal the BF vectors, h1 and h2, to the WTRUs 2101 and 2102, respectively.
FIG. 2 is an example of signaling of CSI or PMI considered in MU MIMO. Even though h1 and h2 represent the beamforming vectors, they can also represent CSI, since the BF vectors are derived from the CSI. In some cases, the CSI may be deduced from the BF vectors. Accordingly, the BF vectors and the CSI may be interchangeable with each other.
In order for each WTRU 210 to perform MU interference cancellation effectively at its receiver, it is desirable for the individual WTRU 210 to also have access to the BF vectors of the other WTRUs 210. To do that, the eNodeB 205 needs to send all of the BF vectors h1 and h2 to each of the WTRUs 210. This causes the signaling overhead to be increased by the number of interfering WTRUs, (i.e., number of paired WTRUs).
Therefore, it is desirable to have efficient control signaling mechanisms using the concept of networking.